AIO for asking my partner to stop talking to someone

My partner and I have been together since December. We actually met through a mutual friend about two years ago. For the sake of the story, I’ll call him Brian.
When we first got together and started telling people, one of the first things several people would say was, “Don’t tell Brian if you want your relationship to last.” At the time, I didn’t think too much of it, but over time I’ve realized that I really can’t trust Brian as a friend.

I’ve talked to my partner about this and told her that I’m uncomfortable with her having a friendship with him because of the things I’ve experienced and because I don’t trust his intentions. She reassured me that I can trust her and that she had handled the situation.

This person has become one of the biggest sources of conflict in our relationship. Today, she was showing me texts from her sister, and I happened to notice that she was still texting Brian. The conversations weren’t flirty or inappropriate from what I saw, so I’m not accusing her of cheating or anything like that.

What bothers me is that she previously told me to trust her and reassured me that she had handled the situation. Now I’m seeing that they’re still communicating, which makes me feel like the issue wasn’t actually handled the way I thought it was.

I’m struggling because I don’t want to be controlling about who she can and can’t talk to, and I know she’s ultimately allowed to have her own friendships. At the same time, I feel like my trust has been affected because this was something we had already discussed, and I feel like the reassurance I was given doesn’t match what I’m seeing now.
Am I overreacting here? Or is it reasonable for me to feel hurt and question the situation, even though the texts themselves weren’t flirty or inappropriate?